Analytical instruments rely on precise control of various active devices that operate in response to control signals from a control system. For example, electronic pneumatic control (EPC) systems are known to offer programmed control of active devices for controlling fluid flow in a fluid bearing conduit in response to sense signals from one or more sensors. The active devices under control of such an EPC system can include valves, switches, and the like. An example of an electronic pressure control system is disclosed, for example, in Henderson, et al., U.S. Pat. No. 5,431,712 and Klein et al., in U.S. Pat. No. 5,108,466.
As shown in FIG. 1, certain operating conditions of the analytical instrument, such as fluid flow through a conduit 11, may be controlled electronically by a pneumatic control system 100. An error amplifier 12 provides a drive signal to an active device 14 such as an electronically-driven proportional valve. The error amplifier 12 provides a pulse-width-modulated drive signal on a control signal line 15 in response to a comparison of a reference level REF and a sensed parameter supplied by a sensor 16. The sensed parameter may be, e.g., fluid pressure or flow rate. The active device 14 operates on electrical power supplied in the form of a regulated voltage V.sub.R on a regulated supply line 18. The active device 14 is actuated according to the drive signal supplied to a buffer 17.
The unregulated supply line 19 can also be subject to wide variations in its voltage level and therefore a power supply 20 is connected to the unregulated supply line 19 so as to provide a regulated supply voltage V.sub.R on a regulated supply line 18. Due to the presence of other active devices (not shown) connected to the unregulated supply line 19, the total load on the unregulated supply line 19 may change abruptly and induce unwanted variations in the unregulated supply voltage V.sub.U. If the power supply 20 does not provide adequate isolation of such variations in the unregulated supply voltage, or is inadequate to meet the demand for, e.g., increased current by the active device 14 or other active devices connected to the regulated supply line 18, the regulated voltage V.sub.R will vary.
Variations in the regulated voltage V.sub.R can be propagated through the regulated supply line 18 to the active device 14. If so, the varying voltage level causes a destabilizing effect on the operation of the pneumatic control system 100 and in particular the servo loop provided by the sensor 16, active device 14, buffer 17, and error amplifier 12. The destabilizing effect can be understood if the response by the active device 14 to a change in the drive signal is considered as a transfer function. In a pulse-width-modulated system, the amount of energy applied to the active device 14 by a given pulse width will depend upon the level of the regulated supply voltage V.sub.R. Thus, any variation in the supply voltage V.sub.DC can cause a change in the open loop gain of the pneumatic control system 100. Further, the overall frequency response of the pneumatic control system 100 may be described in terms of the open loop gain and bandwidth of the servo system; accordingly, changes in the open loop gain will affect the stability of the control loop. The drive signal will change in response to the change in the open loop gain. Additionally, if a change in the regulated supply voltage V.sub.R is faster than the bandwidth of the control loop, there will be a corresponding momentary error in the performance of the system in effecting the desired operating condition.
When there is a change in the regulated supply voltage V.sub.R, the active device 14 responds accordingly, and the active device 14 changes the operating condition in an unintended, erroneous, and undesirable way.
The conventional approach to this problem is to provide an extremely robust power supply 20 to maintain a stable regulated supply voltage. Such a power supply 20 is typically constructed to include heavy-duty one or more linear regulators or a switch mode power converter. However, the regulated voltage provided by a linear regulator is designed to be less than the unregulated supply voltage and therefore significantly less than the maximum level of the unregulated supply voltage V.sub.U. A linear regulator dissipates a substantial amount of power due to the voltage drop between its input terminal to its output terminal. Such dissipation causes undesired heating, excess power consumption, and degradation of the electronic components that make up the linear regulator. A switch mode power converter operates more efficiently and with much less power loss, but has other undesirable features, such as the generation of unwanted radio frequency interference. Switch mode power converters also are generally more complex and expensive to manufacture than would be desired.